First let's define with the help of Wikipedia what a dictionary attack is:
In cryptanalysis and computer security, a dictionary attack is a technique for defeating a cipher or authentication mechanism by trying to determine its decryption key or passphrase by searching likely possibilities.
A dictionary attack uses a brute-force technique of successively trying all the words in an exhaustive list called a dictionary (from a pre-arranged list of values). In contrast with a normal brute force attack, where a large proportion key space is searched systematically, a dictionary attack tries only those possibilities which are most likely to succeed, typically derived from a list of words for example a dictionary (hence the phase dictionary attack) or a bible etc. Generally, dictionary attacks succeed because many people have a tendency to choose passwords which are short (7 characters or fewer), single words found in dictionaries or simple, easily-predicted variations on words, such as appending a digit.
So as you can see, we have two "types of brute force attacks" those which use dictionary and those that does not. With this technique we will be protected from both of them.
This technique, uses iptables to block a particular IP, that has passed the threshold of a certain number of connections in a given period of time.
I will show here, some basic IPtables rules to protect a web server from brute force attacks, but this example can be adapted to other scenarios.
